Charge decay measurement systems and methods

ABSTRACT

Various approaches to can be used to interrogate a surface such as a surface of a layered semiconductor structure on a semiconductor wafer. Certain approaches employ Second Harmonic Generation and in some cases may utilize pump and probe radiation. Other approaches involve determining current flow from a sample illuminated with radiation. Decay constants can be measured to provide information regarding the sample. Additionally, electric and/or magnetic field biases can be applied to the sample to provide additional information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/690,256, filed on Apr. 17, 2015, which claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” which is incorporated by reference herein in its entirety, including but not limited to each of the Sections I, II, III, and IV, which are each incorporated herein by reference in their entirety.

FIELD

The subject filing relates to systems for based wafer inspection, semiconductor metrology, materials characterization, surface characterization and/or interface analysis.

BACKGROUND

In nonlinear optics, light beam input(s) are output as the sum, difference or harmonic frequencies of the input(s). Second Harmonic Generation (SHG) is a non-linear effect in which light is emitted from a material at a reflected angle with twice the frequency of an incident source light beam. The process may be considered as the combining of two photons of energy E to produce a single photon of energy 2E (i.e., the production of light of twice the frequency (2ω) or half the wavelength) of the incident radiation.

A survey of scientific investigations in which the SHG technique has been employed is provided by, “Optical Second-Harmonic Generation from Semiconductor Surfaces” by T. F. Heinz et al., Published in Advances in Laser Science III, edited by A. C. Tam, J. L. Cole and W. C. Stwalley (American Institute of Physics, New York, 1988) p. 452. As reviewed, the SHG process does not occur within the bulk of materials exhibiting a center of symmetry (i.e., in inversion or centrosymmetric materials). For these materials, the SHG process is appreciable only at surfaces and/or interfaces where the inversion symmetry of the bulk material is broken. As such, the SHG process offers a unique sensitivity to surface and interface properties.

So-understood, the SHG effect is described in U.S. Pat. No.5,294,289 to Heinz et al. Each of U.S. Pat. Nos. 5,557,409 to Downer, et al., 6,795,175; 6,781,686; 6,788,405; 6,819,844; 6,882,414 and 7,304,305 to Hunt, 6,856,159 to Tolk, et al. and 7,158,284 to Alles, et al. also describe other approaches or “tools” that may be employed. Yet, the teachings of these patents appear not to have overcome some of the main obstacles to the adoption of SHG as an established technique for use in semiconductor manufacturing and metrology.

SUMMARY

To date, there has been limited adoption of SHG-based metrology tools. It is believed that this fact stems from an inability of existing systems to make distinctions between detected interfacial properties. In other words, while existing SHG techniques offer means of determining location and presence of interfacial electrically active anomalies, their methods rely on relative measurements and are not practically able to parse between electrically active anomaly types (e.g., gettered contaminants such as copper vs. bond voids) and/or to quantify detected contaminants.

However, the subject systems and methods variously enable capturing the quantitative information for making the determinations required for such activity. In these systems and methods, after charging a wafer sample with optical electro-magnetic radiation (at a specific site with a pulsed laser or with a flash lamp or other electro-magnetic energy source or light source or other means) a plurality of measurements are made to monitor transient electric field decay associated with heterointerfaces controlling the decay period.

Using decay curve data generated and characterized with multiple points, spectroscopic parameters of an anomaly or problem at a sample site can be determined such that differentiation and/or quantification of defect type or contaminant(s) is possible. In all, the decay dependent data is collected and used to provide systems by which charge carrier lifetimes, trap energies and/or trapped charge densities may be determined in order that defects and contaminants can be discerned or parsed from one another, for species differentiation if a contaminant is detected and/or for contaminant quantification if detected.

Such activity is determined on a site-by-site basis with the selected methodology typically repeated to scan an entire wafer or other material sample or region thereof. As for the computer processing required to enable such determination, it may occur in “real time” (i.e., during the scanning without any substantial delay in outputting results) or via post-processing. However, in various embodiments, control software can run without lag in order to provide the precise system timing to obtain the subject data per methodology as described below.

Optionally, sample material charge-up is monitored in connection with SHG signal production. In which case, the information gained via this signal may be employed in material analysis and making determinations.

In any case, system embodiments may include an ultra-short pulse laser with a fast shutter operating in the range of 10² seconds to picosecond (10⁻¹² seconds) range. Such systems may be used to monitor SHG signal generation at a sample site from surface and buried interfaces of thin film materials after the introduction of a plurality of short blocking intervals. These intervals may be timed so as to monitor the field decay of interest.

The subject systems may also include an optical line delay. The delay line may be a fiber-based device, especially if coupled with dispersion compensation and polarization control optics. Alternatively, the delay line may be mirror-based and resemble the examples in U.S. Pat. Nos. 6,147,799 to MacDonald, 6,356,377 to Bishop, et al. or 6,751,374 to Wu, et al. In any case, the delay is used in the system in order to permit laser interrogation of the material in the picosecond (10⁻¹² second) to femtosecond (10⁻¹⁵ second) and, possibly, attosecond (10⁻¹⁸ second) ranges. Such interrogation may be useful in detecting multiple charge decay-dependent data points along a single decay curve.

The subject methods include one that involves measuring an SHG signal for decay data points acquired after successive charge-up events. The conditions for obtaining a SHG signal may be different at each charge-up event. Additionally, the time interval between successive charge-up events may be different. In this method, the multiple data points (at least two but typically three or more) can be correlated and expressed as a single composite decay curve. Another method employs minimally disruptive (i.e., the radiation used to produce the SHG signal does not significantly recharge the material) SHG signal interrogation events after a single charging event.

Yet another method for determining transient charge decay involves measuring discharge current from the sample material (more accurately, its structures that were charged by optical radiation). The time dependence (kinetics) of this signal may then be treated in the same way as if SHG sensing had been employed. Further, as above, such sensing may be done in the span of one decay interval and/or over a plurality of them following charge to a given level. In any case, electrode-specific hardware for such use is detailed below.

Regarding charge or charging level, this may be taken to a point of apparent saturation when charge dynamics are observed in standard linear time or against a log time scale. Per above, the subject methodologies optionally observe, record and analyze charging kinetic as this may yield important information.

For successive charge/interrogation events, if an initial charge state of a sample is measured and the saturation level is not far from the initial charge state, the system may omit further or subsequent characterization. In this context, what may be regarded as “not far” may mean about 1% to about 10% of charge increase versus the initial charge state to be determined by learning when the subject tool is used for a given time of sampling.

Stated otherwise, so-called “saturation” is a relative term. Using a linear time scale, material will appear saturated very quickly. But if an SHG signal intensity associated with charging is observed in log scale from 10-100 seconds, it can be observed that the later part of saturation occurs with a different time constant and is relatively more gradual or time-consuming. Thus, while examples of the methodology provided herein discuss charging to saturation, the delay and other timing may be regarded as occurring with respect to apparent saturation. Rather than waiting the full amount of time for 100% saturation, as this may be unnecessarily time consuming to reach, instead, the instrument may delay until the time it takes to get to apparent saturation or the time in which can extract important parameters, regardless of how long it takes for full saturation.

Further, it is to be understood that when monitoring the amount or degree of charge-up toward saturation (e.g., in connection with SHG monitoring), the subject methods and systems may operate with charge and/or re-charging levels at less than saturation (as discussed above) while still yielding meaningful decay curve information. Without such measurement, however, when approximate saturation is a known parameter (e.g., by experience with the subject tool with a given material) charge to saturation is employed as the target level.

Notably, various interfacial material properties may also be determined using laser beam blocking or delay as further described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section III, titled “TEMPERATURE-CONTROLLED METROLOGY,” which is incorporated herein by reference in its entirety. Introducing a DC bias across the sample being tested can also assist in analysis of the material. Employing a DC bias actively changes the initial charge distribution at the interfaces before photo-induced voltage has any effect. To do so, the sample being tested may be mounted atop a conductive chuck which can be used as a ground for DC biasing across the sample using sample top surface probes. Other means of introducing induced voltage biases are possible as well without the use of surface probes as further described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section IV entitled, “FIELD-BIASED SHG METROLOGY,” which is incorporated herein by reference in its entirety. See also co-pending U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “FIELD-BIASED SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, which is incorporated herein by reference in its entirety.

Also, the subject systems may use a secondary light source in addition to the primary laser involved in blocking-type analysis for charge decay determination. Such a set of sources may be employed as a radiation pump/probe combination as further described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section I entitled, “PUMP AND PROBE TYPE SHG METROLOGY,” which is incorporated herein by reference in its entirety. See also co-pending U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “PUMP AND PROBE TYPE SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, which is incorporated herein by reference in its entirety.

All said, invention embodiments hereof include each of the methodology associated with the approaches described herein, alone or in combination with elements components or features from the referenced co-pending patent applications and/or from the disclosure incorporated herein by reference, hardware to carry out the methodology, productions systems incorporating the hardware and products (including products-by-process) thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures diagrammatically illustrate aspects of various embodiments of different inventive variations.

FIGS. 1A-1C are diagrams of systems embodiments;

FIG. 2 is a chart of system function; FIGS. 3A and 3B are charts representative of the manner of delivering such function; FIG. 4 represents system function in a graphical output.

FIGS. 5 and 6 plot SHG interrogation-related method embodiments; FIGS. 7A-7E plot time dynamics associated with the system in FIG. 1C that may be employed in the methods of FIGS. 5 and 6.

FIG. 8 plots a current-based interrogation method for observing transient electric field decay; FIGS. 9A and 9B illustrate hardware configurations that may be employed in the method of FIG. 8.

DETAILED DESCRIPTION

FIG. 1A is a diagram of a first system 2100 as may employed in connection with the subject methodology. Alternative systems 2100′ and 2100″ are shown in FIGS. 1B and 1C. Each system includes a primary laser 2010 for directing a primary beam 2012 of electro-magnetic radiation at a sample wafer 2020, which sample is held by a vacuum chuck 2030. The chuck 2030 includes or is set on x- and y-stages and optionally also a rotational stage for positioning a sample site 2022 across the wafer relative to where the laser(s) are aimed. A beam 2014 of reflected radiation directed at a detector 2040 will include an SHG signal. The detector may be any of a photomultiplier tube, a CCD camera, a avalanche detector, a photodiode detector, a streak camera and a silicon detector. The sample site 2022 can include one or more layers. The sample site 2022 can comprise a composite substrate including at least two layers. The sample site 2022 can include an interface between two dissimilar materials (e.g., between two different semiconductor materials, between two differently doped semiconductor materials, between a semiconductor and an oxide, between a semiconductor and a dielectric material, between a semiconductor and a metal, between an oxide and a metal, between a metal and a metal or between a metal and a dielectric).

Also common to each of the embodiments is the inclusion of one or more shutter-type devices 2050. These are employed as described in connection with the methodology below. The type of shutter hardware used will depend on the timeframe over which the laser radiation is to be blocked, dumped or otherwise directed away from the sample site.

An electro-optic blocking device such as a Pockel's Cell or Kerr Cell is used to obtain very short blocking periods (i.e., with switching times on the order of 10⁻⁹ to 10⁻¹² seconds). For longer blocking time intervals (e.g., from about 10⁻⁵ seconds and upwards) mechanical shutters or flywheel chopper type devices may be employed.

However, electro-optic blocking devices will allow a wider range of materials to be tested in accordance with the methods below. A photon counting system 2044 capable of discretely gating very small time intervals, typically, on the order of picoseconds to microseconds can be included to resolve the time-dependent signal counts.

Hardware is contemplated for pushing the methods into faster-yet time frames. Namely, as shown in FIG. 1C, the system(s) may include delay line hardware 2060. Beam splitting and switching (or shuttering on/off) between a plurality of set-time delay lines for a corresponding number of time-delayed interrogation events is possible. However, a variable delay line may be preferred as offering a single solution for multiple transient charge decay interrogation events on a time frame ranging from immediately (although delay of only 10⁻¹² seconds may be required for many methodologies) to tens of nanoseconds after pump pulse. The desired delay time may even go into the microsecond regime if using a slower, kilohertz repetition laser. And while such hardware is uniquely suited for carrying out the subject methodology (both of which methodology and such hardware is believed heretofore unknown), it might be put to other uses as well.

In the implementation illustrated in FIG. 1C, the beam 2012 from the laser 2010 can be split by a beam splitter 2070 between two optical paths. The beam splitter 2070 can split the beam 2012 unequally between the two optical paths. For example, 70% of the energy of the beam 2012 can be directed along a first optical path (e.g., as beam 2016) and 30% of the energy of the beam 12 can be directed along a second optical path (e.g., as beam 2018). As another example, 60% of the energy of the beam 2012 can be directed along the first optical path and 40% of the energy of the beam 2012 can be directed along the second optical path. As yet another example, 80% of the energy of the beam 2012 can be directed along the first optical path and 20% of the energy of the beam 2012 can be directed along the second optical path. The beam splitter 2070 can comprise a dielectric mirror, a splitter cube, a metal coated mirror, a pellicle mirror or a waveguide splitter. In implementations, where the beam 2012 includes optical pulses, the beam splitter 2070 can include an optical component having negligible dispersion that splits the beam 2012 between two optical paths such that optical pulses are not broadened. As indicated by the double-arrow in FIG. 1C, the path of an “interrogation” beam 2016 taken off a beam splitter 2070 from primary beam 2012 can be lengthened or shortened to change its arrival timing relative to a “pump” beam 2018 wherein each of the beams are shown directed or aimed by various mirror elements 2072. Another approach (mentioned above) employs fiber optics in the optical delay component and/or other optical pathways (e.g., as presented in U.S. Pat. No. 6,819,844 incorporated herein by reference in its entirety for such description).

The output from the detector 2040 and/or the photon counting system 2044 can be input to an electronic device 2048 (see, e.g., FIGS. 1A and 1B). The electronic device 2048 can be a computing device, a computer, a tablet, a microcontroller or a FPGA. The electronic device 2048 includes a processor or processing electronics that may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application. The electronic device 2048 can implement the methods discussed herein by executing instructions included in a machine-readable non-transitory storage medium, such as a RAM, ROM, EEPROM, etc. The electronic device 2048 can include a display device and/or a graphic user interface to interact with a user. The electronic device 2048 can communicate with one or more devices over a network interface. The network interface can include transmitters, receivers and/or transceivers that can communicate over wired or wireless connections.

Another potential aspect of system 2100″ concerns the manner in which the initial beam splitter works. Namely, the split may be unequal (e.g., 70-30%, 80-20%, 60-40% or any range therebetween, such as between 60-90% in one path and between 40-10% in another path as well as outside these ranges), sending a majority of the power in the pump beam, and a minority in the probe beam. For example, the split may be 60-70% and 40-30%, for the pump and probe, respectivley, 70-80% versus 30-20% for the pump and probe, respectively, 80-90% versus 20-10%, for the pump and probe respectively, or 90-99.999% versus 10-0.001%, for the pump and probe respectively,. In different embodiments, the probe beam could be between 0.001% to 49.99% while the pump beam could be between 50.001% and 99.999%, for example. The sum of the two beams may be 100% or approximate thereto. The split may be determined by the particular material system being characterized in some cases. The value (at least in part) of doing so may be to help facilitate methods such as shown in FIGS. 5 and 6 in which the power involved in SHG interrogation subsequent to material charging is desirably reduced or minimized as discussed below. Still another aspect is that the pump and probe beams are brought in at different angles. Such an approach facilitates measuring pump and probe SHG responses separately. In such cases, two detectors may be advantageously employed with one for each reflected beam path.

Various other optional optics distinguish the embodiments shown. For example, embodiments 2100 and 2100′ are shown including a dichroic reflective or refractive filter 2080 for selectively passing the SHG signal coaxial with reflected radiation directly from the laser 2010. Alternatively, a prism may be employed to differentiate the weaker SHG signal from the many-orders-of-magnitude-stronger reflected primary beam. However, as the prism approach has proved to be very sensitive to misalignment, a dichroic system as referenced above may be preferred. Other options include the use of diffraction grating or a Pellicle beam splitter. As shown in system 2100, an optical bundle 2082 of focusing and collimating/collimation optics may be provided. As shown in system 2100′, a filter wheel 2084, zoom lens 2086 and/or polarizers 2088 may be employed in the system(s). Also, an angular (or arc-type) rotational adjustment (with corresponding adjustment for the detector 2040 and in-line optical components) as shown in system 2100′ may be desirable. An additional radiation source 2090 (be it a laser illustrated emitting a directed beam 2092 or a UV flash lamp emitting a diverging or optically collimated or a focused pulse 2094) may also be incorporated in the system(s) to provide such features as referenced above in connection with the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section I entitled “PUMP AND PROBE TYPE SHG METROLOGY,” which is incorporated herein by reference in its entirety and/or initial charging/saturation in the methods below. See also co-pending U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “PUMP AND PROBE TYPE SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, which is incorporated herein by reference in its entirety.

In these systems, laser 10 may operate in a wavelength range between about 700 nm to about 2000 nm with a peak power between about 10 kW and 1 GW, but delivering power at an average below about 100 mW. In various embodiments, average powers between 10 mW and 10 W should be sufficient. Additional light source 2090 (be it a another laser or a flash lamp) may operate in a wavelength range between about 80 nm and about 800 nm delivering an average power between about 10 mW and 10 W. Values outside these ranges, however, are possible.

Regarding other system options, since an SHG signal is weak compared to the reflected beam that produces it, it may be desirable to improve the signal-to-noise ratio of SHG counts. As photon counting gate times decrease for the blocking and/or delay processes described herein, improvement becomes even more useful. One method of reducing noise that may be employed is to actively cool the detector. The cooling can decreases the number of false-positive photon detections that are generated randomly because of thermal noise. This can be done using cryogenic fluids such as liquid nitrogen or helium or solid state cooling through use of a Peltier device. Others areas of improvement may include use of a Marx Bank Circuit (MBC) as relevant to shutter speed.

These improvements may be applied to any of the systems in FIGS. 1A-1C. Likewise, any or all of the above features described above in connection with systems 2100 and 2100′ may be incorporated in system 2100″. Indeed a, mix-and-match of features or components is contemplated between all of the systems.

With such systems running the subject methodology, various determinations can be made not heretofore possible using laser-blocking and/or delay related techniques. FIG. 2 illustrates a process map or decision tree 2200 representing such possibilities. Namely, a so-called problem 2210 that is detected can be parsed between a defect 2210 (extended defects such as bond voids or dislocations, Crystal Originated Particle (COP) or the like) and a contaminant 2220 (such as copper inclusion or other metals in point defect or clustered forms). In terms of a defect, the defect type 2222 and/or a defect quantification 2224 determination (e.g., in terms of density or degree) can also be made. In terms of a contaminant, the contaminant species or type 2232 and/or a contaminant quantification 2234 determination can be made. Such parsing between defect and contaminant and identification of species may be performed in connection with determining charge carrier lifetimes, trap energies, trap capture cross-section and/or trap densities then comparing these to values in look-up tables or databases. Essentially these tables or databases include listings of properties of the material as characterized by the subject methods, and then matching-up the stated properties with entries in a table or database that correspond to particular defects or contaminants.

Trap capture cross-section and trap density may be observed in connection with, optionally, detected charging kinetics. As for determining charge carrier lifetimes and trap energies, the following equation based on work by I. Lundstrom, provides guidance:

$\tau = {\tau_{0}\exp \left\{ {\frac{4}{3\; \hslash}{{\sqrt{2\; {em}_{ox}^{*}}\left\lbrack {\varphi_{T}^{3/2} - \left( {\varphi_{T} - {E_{ox}d_{T}}} \right)^{3/2}} \right\rbrack}/E_{ox}}} \right\}}$

where τ is the tunneling time constant for the tunneling mechanism of the trap discharge, ϕ_(r) denotes the trap energy, E_(ox) denotes the strength of the electric filed at the interface and the remaining equation variables and context are described at I. Lundstrom, JAP, v.43,n. 12,p.5045, 1972 which subject matter is incorporated by reference in its entirety. Further modeling and calculation options may be appreciated in reference to the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” referred to Section III, titled “TEMPERATURE-CONTROLLED METROLOGY,” which is incorporated herein by reference in its entirety.

In any case, the decay curve data obtained by the subject sample interrogation can be used to determine the parameters of trap energy and charge carrier lifetime by use of physical models and related mathematics. Representative sets of curves 2300, 2300′ such as those pictured in FIGS. 3A and 3B may be calculated (where FIG. 3B highlights or expands a section of the data from FIG. 3A) from the equation above.

These curves demonstrate the relationship between time constant (vertical axis) and dielectric thicknesses (horizontal axis) for different trap or barrier energies. The vertical axis includes the ultrafast time scales of down to nanoseconds (1E-9s)). The horizontal axes are tunneling distances (or dielectric thickness, both terms being generally equivalent in this example). The different curves are lines of constant barrier energy. For example, in FIG. 3B, an electron caught in a trap with an energy depth of the listed barrier energy of 0.7 eV would exhibit a detrapping time constant of about 1E-5 seconds if the dielectric thickness was 40 Angstroms.

Further modeling with Poisson/Transport solvers can be used to determine trap density in MOS-like structures and more exotic devices using charge carrier lifetimes and known trap energies. Specifically, the photo-injected current due to femto-second optical pulses induces bursts of charge carriers which reach the dielectric conduction band. The average value of this current can be related to carrier concentration and their lifetimes in the regions. The E-field across the interface is the proxy by which SHG measures these phenomena.

In the plot of FIG. 3A, it can be observed (see dashed lines) that 20 Angstrom of oxide has 1 msec discharge time constant for a trap having an energy of about 3 eV To relate the plots to an example of use in the subject system, suppose a 20 Angstrom oxide is interrogated after blocking laser excitation. As shown in FIG. 3B (see highlighted box) the result will be observable current from 1 μsec to about 1 msec and then all the current dies out.

The decay curves discussed in this application can be a product of multiple processes (e.g., charge relaxation, charge recombination, etc.) from traps having different energies and different relaxation/recombination time constants. Nevertheless, in various embodiments, the decay curves can be generally expressed by an exponential function f(t)=Aexp(−λt)+B, where A is the decay amplitude, B denotes the baseline offset constant and λ denotes a decay constant. This general exponential function can be used to approximately characterize the “extent of decay” from experimentally obtained decay data curve. In various embodiments, it is possible to use the half-life t_(1/2), average lifetime τ, and decay constant λ, to characterize the extent of decay for a decay curve (obtained experimentally or by simulation). For example, the parameters A, B, and λ can be obtained from the decay data points that are obtained experimentally as discussed below. An average lifetime τ can then be calculated from the parameters A, B, and λ using theory of radioactive decay as a way of setting benchmarks for what is qualitatively called partial, or full-decay. For example, in some embodiments, τ can be given by the equation (t_(1/2))/(ln(2)).

In various implementations, the charge state can be considered to have fully decayed after a time span of three average lifetimes τ, which corresponds to ˜95% decay from full saturation. Partial decay can be expressed in terms of signal after a certain number of average lifetimes τ have elapsed.

In operations, the systems determine parameters (e.g., carrier lifetimes, trap energies, trapping cross-section, charge carrier density, trap charge density, carrier injection threshold energy, charge carrier lifetime, charge accumulation time, etc.) based at least in part on the subject methodology on a point-by-point basis on a portion (e.g., die size portion)of the wafer or an entire wafer. An entire wafer (depending on the material, surface area, and density of scan desired) can often be scanned in less than about 10 minutes, with these parameters determined for each point scanned. In various embodiments, a location of the wafer can be scanned in a time interval between about 100 milliseconds and about 3 seconds. For example, a location of the wafer can be scanned in about 950 milliseconds.

A matrix of data containing the spatial distributions of the parameters determined can be plotted as individual color-coded heat maps or contour maps for each parameter, as a means for quantitative inspection, feedback and presentation. FIG. 4 illustrates one such map 2400. It depicts how a defect 2402 may be portrayed. But it is possible to show any of the further refined subject matters in FIG. 2. Once quantitative data has been obtained, providing such output is merely a matter of changing the code in the plotting program/script.

Such information and/or other information treated below may be shown on a computer monitor or dedicated system display and/or it may be recorded for later reference or use for analysis on digital media. In addition, each wafer spatial distribution can be cross-correlated by referencing with ellipsometry data to correct for layer thickness variability and cross-calibrated with independent contamination characterization data obtained, for example, by Total Reflection X-ray Fluorescence (TXRF), Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and the like. These initial or corrected spatial distributions can then be compared to those from wafers known to be within specification, to determine if the samples in question have any defects or problematic features which warrant further testing. In general, however, it is desirable to use low-cost SHG and other methods hereof calibrated with, by or against slow and expensive direct methods like TXRF, etc.

Human decisions may be employed (e.g., in inspecting a generated heat map 2400) initially in determining the standard for what is an acceptable or unsatisfactory wafer, until the tool is properly calibrated to be able to flag wafers autonomously. For a well-characterized process in a fab, human decisions would then only need to be made to determine the root cause of any systemic problem with yields, based on the characteristics of flagged wafers.

However implemented, FIG. 5 provides a plot 2500 illustrating a first method embodiment hereof that may be used in making such determinations. This method, like the others discussed and illustrated below relies on characterizing SHG response with multiple shutter blocking events in which interrogation laser is gated for periods of time.

In this first example, a section of a sample to be interrogated is charged (typically by a laser) to saturation. In this example, a single source is used to generate as pump beam and probe beam, although separate pump and probe sources can be used in other embodiments. During which time, the SHG signal may be monitored. The saturation level may be known by virtue of material characterization and/or observing asymptotic behavior of the SHG signal intensity associate with charging (I_(ch)). Upon (or after) reaching saturation, the electromagnetic radiation from the laser (pump beam) is blocked from the sample section. The laser (probe beam) is so-gated for a selected period of time (t_(bl1)). After gating ceases, an SHG intensity measurement (I_(dch1)) is made with the laser (probe beam) exposing the surface, thus observing the decay of charge at a first discharge point. After charging the material section (with the pump beam) to saturation again over a period of time (t_(ch)), a second blocking event occurs for a time (t_(bl2)) different than the first in order to identify another point along what will become a composite decay curve. Upon unblocking the laser (probe beam), SHG signal intensity (I_(dchs2)) is measured again. This reduced signal indicates charge decay over the second gating event or blocking interval. Once-again charged to saturation by the laser (pump beam), a third differently-timed blocking event (t_(bl3)) follows and subsequent SHG interrogation and signal intensity measurement (I_(dch3)) is made for a third measurement of charge decay in relation to SHG intensity.

Although in the above example, the sample is charged to a saturation level, in other examples, the sample can be charged to a charge level below saturation. Although in the above example, the three blocking times t_(bl1), t_(bl2) and t_(bl3) are different, in other examples, the three blocking times t_(bl1), t_(bl2) and t_(bl3) can be the same. In various examples, the sample can be charged to a charging level initially and the SHG intensity measurement (I_(dch1)), (I_(dch2)) and (I_(dch3)) can be obtained at different time intervals after the initial charging event.

As referred to above, these three points (corresponding to L_(dch1), I_(dch2) and I_(dch3)) can be used to construct a composite charge decay curve. It is referred to herein as a “composite” curve in the sense that its components come from a plurality of related events. And while still further repetition (with the possibility of different gating times employed to generate more decay curve data points or the use of same-relation timing to confirm certainty and/or remove error from measurements for selected points) may be employed so that four or more block-then-detect cycles are employed, it should be observed that as few as two such cycles may be employed. Whereas one decay-related data point will not offer meaningful decay curve characterization, a pair defining a line from which a curve may be modeled or extrapolated from to offer some utility, whereas three or more points for exponential decay fitting will yield an approximation with better accuracy. Stated otherwise, any simple (e.g., not stretched by dispersive transport physics) decay kinetics has a general formula: Measurable(t)=M₀*exp(−t/tau) so to find two unknown parameter M₀ and tau at least 2 points are needed assuming this simple kinetics. In dispersive (i.e., non-linear) kinetics it is desirable to measure as many point as possible to extract (n-1)-order correction parameters if n-points are measured and then apply a model appropriate for that order of approximation. Also, that set of measurements is to be measured for different electric fields (E) to be real practical and precise with the tau to assign it for a certain type of defects.

The method above can provide parameter vs. time (such as interfacial leakage current or occupied trap density v. time) kinetic curve by obtaining measurements at a few time points. A time constant (τ) can be extracted from the parameter vs. time kinetic curve. The time constant can be attributed to a time constant characteristic for a certain type of defect.

In any case, the decay-dependent data obtained may be preceded (as in the example) by SHG data acquisition while saturating the material with the interrogation (or probe) laser. However, charging will not necessarily go to saturation (e.g., as noted above). Nor will the measurement necessary be made prior to the blocking of a/the charging laser. Further, the charging will not necessarily be performed with the interrogation/probe laser (e.g., see optional pump/probe methodology cited above).

Regardless, after the subject testing at one sample site, the sample material is typically moved or indexed to locate another section for the same (or similar) testing. In this manner, a plurality of sections or even every section of the sample material may be interrogated and quantified in scanning the entire wafer as discussed above.

FIG. 6 and plot 2600 illustrate an alternative (or complimentary approach) to acquiring charge decay related data by scanning is shown in plot 2600. In this method, after charging to saturation a/the first time, continuous (or at least semi-continuous) discharge over multiple blocking time intervals (t_(bl1), t_(bl2), t_(bl3)) is investigated by laser pulses from an interrogation or a probe laser measuring different SHG intensities (I_(dch1), I_(dch2), I_(dch3)). The intensity and/or frequency of the laser pulses from the interrogation/probe laser are selected such that the average power of the interrogation/probe laser is reduced to avoid recharging the material between blocking intervals while still obtaining a reasonable SHG signal. To do so, as little as one to three laser pulses may be applied. So-reduced (in number and/or power), the material excitation resulting from the interrogation or probe laser pulses may be ignored or taken into account by calibration and or modeling considerations.

In various embodiments, a separate pump source can be used for charging. However, in some embodiments, the probe beam can be used to charge the sample.

In any case, the delay between pulses may be identical or tuned to account for the expected transient charge decay profile or for other practical reason. Likewise, while the delay is described in terms of “gating” or “blocking” above, it is to be appreciated that the delay may be produced using one or more optical delay lines as discussed above in connection with FIG. 1C. Still further, the same may hold true for the blocking/gating discussed in association with FIG. 5.

Further, as above, the method in FIG. 6 may be practiced with various modifications to the number of blocking or delay times or events. Also, SHG signal may or may not be measured during charge to saturation. Anyway, the method in FIG. 6 may be practiced (as illustrated) such that the final gating period takes the SHG signal to null. Confirmation of this may be obtained by repeating the method at the same site in a mode where charging intensity (I_(ch)) is measured or by only observing the SHG signal in (re)charging to saturation.

FIGS. 7A-7E are instructive regarding the manner in which the subject hardware is used to obtain the decay-related data points. FIG. 7A provides a chart 2700 illustrating a series of laser pulses 2702 in which intermediate or alternating pulses are blocked by shutter hardware (e.g., as described above) in a so-called “pulse picking” approach. Over a given time interval, it is possible to let individual pulses through (indicated by solid line) and block others (as indicated by dashed line).

FIG. 7B provides a chart 2710 illustrating the manner in which resolution of a blocking technique for SHG investigation can be limited by the repetition (rep) rate of the probe laser. Specifically, when presented with a decay curve like decay curve 2712 it is possible to resolve the time delay profile with blocking of every other pulse using a pulsed laser illustrated to operate at the same time scale as in FIG. 7A. However, a shorter curve 2714 cannot be resolved or observed under such circumstances. As such, use optical delay stage(s) can offer additional utility.

Accordingly, chart 2720 in FIG. 7C illustrates (graphically and with text) how blocking and introducing a delay with respect to a reference time associated with charging the sample can offer overlapping areas of usefulness, in terms of the decay time of the curve relative to the rep rate of the laser. It also shows how there are short time ranges when only delay stages would allow interrogation of the decay curve, and longer time ranges when only blocking the pumping and/or the probing beam would be practical.

FIGS. 7D and 7E further illustrate the utility of the combined block/delay apparatus. Chart 2730 illustrates exemplary SHG signals produced by individual laser pulses 2702. With a delay stage alone, only the range (X) between each such pulse may be interrogated by varying optical delay. In contrast, additional utility over a range (Y) may be achieved with a system combining a delay stage and blocking or shutter means such as a chopper, shutter, or modulator. As illustrated by chart 2740, such a system is able to measure decay curves (and their associated time constants) in the range from one to several pulse times.

FIG. 8 provides a plot 2800 illustrating a third method embodiment hereof. This embodiment resembles that in FIG. 6, except that discharge current (J_(dch1), J_(dch2), J_(dch3)) is measured at time intervals (e.g., t_(i)=t₀, 2t₀, 3t₀, 7t₀, 10t₀, 20t₀, 30t₀, 70t₀—basically according to a log time scale vs. linear time—where t₀ is a scale parameter of about 10⁻⁶ sec or 10⁻³ sec when measurement is started) after charging the material with a laser (optionally monitoring or capturing its SHG intensity (Ich) signal) or other electro-magnetic radiation source and then blocking or otherwise stopping the laser radiation application to the sample, thereby allowing discharge. This approach gives an estimation of the mobile carrier lifetime in the substrate by the moment after the e-h-plasma in the substrate is decayed and when the discharge current starts to be seen, thus offering an important physical parameter of the wafer. And after carrier lifetime is determined, the discharge of current can be interpreted in its time dependence (i.e., its kinetics regarding charge decay) in the same manner as if it were obtained by SHG sensing of discharged charge.

Various embodiments can be used to measure time constant (e.g., for decay) having a range of values. For example, the time constants can range between 0.1 femtosecond and 1 femtosecond, 1 femtosecond and 10 femtoseconds, 10 femtoseconds and 100 femtoseconds, 100 femtoseconds and 1 picosecond, between 1 picosecond and 10 picoseconds, between 10 picoseconds and 100 picoseconds, between 100 picoseconds and 1 nanosecond, between 1 nanosecond and 10 nanoseconds, between 10 nanosecond and 100 nanoseconds, between 100 nanoseconds and 1 microsecond, between 1 nanoseconds and 100 microseconds, between 100 microseconds and 1 millisecond, between 1 microsecond and 100 milliseconds, between 100 microsecond and 1 second, between 1 second and 10 seconds, or between 10 second and 100 seconds or larger or smaller. Likewise, time delays (A) for example between the probe and pump (or pump and probe) can be, for example, between 0.1 femtosecond and 1 femtosecond, 1 femtosecond and 10 femtoseconds, 10 femtoseconds and 100 femtoseconds, 100 femtoseconds and 1 picosecond, between 1 picosecond and 10 picoseconds, between 10 picoseconds and 100 picoseconds, between 100 picoseconds and 1 nanosecond, between 1 nanosecond and 10 nanoseconds, between 10 nanosecond and 100 nanoseconds, between 100 nanoseconds and 1 microsecond, between 1 nanoseconds and 100 microseconds, between 100 microseconds and 1 millisecond, between 1 microsecond and 100 milliseconds, between 100 microsecond and 1 second, between 1 second and 10 seconds, between 10 second and 100 seconds. Values outside these ranges are also possible.

Various physical approaches can be taken in providing a system suitable for carrying out the method in FIG. 8—which method, notably, may be modified like those described above. Two such approaches are illustrated in FIGS. 9A and 9B.

Systems 2900 and 2900′ use gate electrodes 2910 and 2920, respectively, made of a conductive material that is transparent in the visible light range. Such an electrode may touch a wafer 2020 to be inspected, but need not as they may only be separated by a minimal distance. In various implementations, the electric field in the dielectric can be estimated by extracting the electrode-dielectric-substrate structure parameters using AC measurement of the Capacitance-Voltage curve (CV-curve). CV-curve measurement can be done by using a standard CV-measurement setup available on the market, connected to a material sample in the subject tool (e.g., the applied voltage is to provide the electric field in the dielectric between about 0.1 MV/cm and about 5 MV/cm). The wafer may be held upon a conductive chuck 2030 providing electrical substrate contact. Another alternative construction for a gate electrode would be an ultra-thin Au film or Al film on a glass of 10-30A thickness which can reduce the sensitivity due to absorption of some photons by the thin semi-transparent metal layer.

However, electrodes 2910 and 2920 present no appreciable absorption issues (although some refraction-based considerations may arise that can be calibrated out or may be otherwise accounted for in the system). These electrodes may comprise a transparent conductor gate layer 2930 made of a material such as ZnO, SnO or the like connected with an electrical contact 2932. An anti-reflective top coat 2934 may be included in the construction. Gate layer 2930 may be set upon a transparent carrier made 2936 of dielectric (SiO₂) with a thickness (D_(gc)) as shown. In various embodiments, the transparent carrier comprises an insulator that is used as a gate for a noncontact electrode that may employ for example capacitive coupling to perform electrical measurements, similar to those described in the portion of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section IV entitled, “FIELD-BIASED SHG METROLOGY,” which is incorporated herein by reference in its entirety. See also co-pending U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “FIELD-BIASED SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, which is incorporated herein by reference in its entirety. As the wafer is charged from the incoming laser radiation, the electric field across one or more of its interfaces will change, and the layers of the wafer should capacitavely couple with the plates in the electrode similar to a plate capacitor. The charging of the electrode will involve movement of charge carriers that will be measured as current.

D_(gc) would be calibrated by measuring CV curve on the semiconductor substrate with a non-invasive approach and used in electric field (E) calculation when applied voltage is known. A negligible gap distance between the gate and sample can be an air gap. Alternatively the electrode can be directly in contact with the sample rather than being separated by an air gap or dielectric. Accordingly normal CV or IV measurements may be performed in various embodiments.

Or given a close refractive index match between water and SiO₂, filling the gap with deionized water may be helpful in reducing boundary-layer reflection without any ill effect (or at least one that cannot be addressed). Deionized (or clean-room grade) water can maintain cleanliness around the electrically sensitive and chemically pure substrate wafers. Deionized water is actually less conductive than regular water.

In FIG. 9B, a related construction is shown with the difference being the architecture of the carrier or gate-holder 2938. Here, it is configured as a ring, optimally formed by etched away in the center and leaving material around the electrode perimeter as produced using MEMS techniques. But in any case, because of the large unoccupied zone through with the laser and SHG radiation must pass, it may be especially desirable to fill the same with DI water as described above.

Regardless, in the overall electrode 2910, 2920 constructions each embodiment would typically be stationary with respect to the radiation exciting the material in use. Prior to and after use, the electrode structure(s) may be stowed by a robotic arm or carriage assembly (not shown).

As describe above, in various embodiments the electrode directly contacts the wafer to perform electrical measurements such as measuring current flow. However, non-contact methods of measuring current, such as for example using electrodes that are capacatively coupled with the sample, can also be used.

The systems and methods described herein can be used to characterize a sample (e.g., a semiconductor wafer or a portion thereof). For example, the systems and methods described herein can be used to detect defects or contaminants in the sample as discussed above. The systems and methods described herein can be configured to characterize the sample during fabrication or production of the semiconductor wafer. Thus, the systems and methods can be used along a semiconductor fabrication line in a semiconductor fabrication facility. The systems and methods described herein can be integrated with the semiconductor fabrication/production line. The systems and methods described herein can be integrated into a semiconductor fab line with automated wafer handling capabilities. For example, the system can be equipped with an attached Equipment Front End Module (EFEM), which accepts wafer cassettes such as a Front Opening Unified Pod (FOUP). Each of these cassettes can be delivered to the machine by human operators or by automated cassette-handling robots which move cassettes from process to process along fabrication/production line.

In various embodiments, the system can be configured such that once the cassettes are mounted on the EFEM, the FOUP is opened, and a robotic arm selects individual wafers from the FOUP and moves them through an automatically actuated door included in the system, into a light-tight process box, and onto a bias-capable vacuum chuck. The chuck may be designed to fit complementary with the robotic arm so that it may lay the sample on top. At some point in this process, the wafer can be held over a scanner for identification of its unique laser mark.

Accordingly, a system configured to be integrated in a semiconductor fabrication/assembly line can have automated wafer handling capability from the FOUP or other type of cassette; integration with an EFEM as discussed above, a chuck designed in a way to be compatible with robotic handling, automated light-tight doors which open and close to allow movement of the robotic wand/arm and software signaling to EFEM for wafer loading/unloading and wafer identification.

As described above, each of Sections I, II, III, and IV of U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGY TECHNOLOGIES,” are incorporated herein by reference in their entirety. Similarly, co-pending patent applications (i) U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “PUMP AND PROBE TYPE SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, and (ii) U.S. patent application Ser. No. ______, filed Apr. 17, 2015 titled “FIELD-BIASED SECOND HARMONIC GENERATION METROLOGY”, published as U.S. Publication No. ______, are each incorporated herein by reference in their entirety. PCT Application No. PCT/US2015/026263, filed Apr. 16, 2015 titled “WAFER METROLOGY TECHNOLOGIES” is also incorporated herein by reference in its entirety. Accordingly, features from the disclosure of any of these documents incorporated by reference may be combined with any features recited elsewhere herein.

Variations

Exemplary invention embodiments, together with details regarding a selection of features have been set forth above. As for other details, these may be appreciated in connection with the above-referenced patents and publications as well as is generally known or appreciated by those with skill in the art. The same may hold true with respect to method-based aspects of the invention in terms of additional acts as commonly or logically employed. Regarding such methods, including methods of manufacture and use, these may be carried out in any order of the events which is logically possible, as well as any recited order of events. Furthermore, where a range of values is provided, it is understood that every intervening value, between the upper and lower limit of that range and any other stated or intervening value in the stated range is encompassed within the invention. Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein.

Though the invention embodiments have been described in reference to several examples, optionally incorporating various features, they are not to be limited to that which is described or indicated as contemplated with respect to each such variation. Changes may be made to any such invention embodiment described and equivalents (whether recited herein or not included for the sake of some brevity) may be substituted without departing from the true spirit and scope hereof

The various illustrative processes described may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor can be part of a computer system that also has a user interface port that communicates with a user interface, and which receives commands entered by a user, has at least one memory (e.g., hard drive or other comparable storage, and random access memory) that stores electronic information including a program that operates under control of the processor and with communication via the user interface port, and a video output that produces its output via any kind of video output format, e.g., VGA, DVI, HDMI, DisplayPort, or any other form.

A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. These devices may also be used to select values for devices as described herein.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

In one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on, transmitted over or resulting analysis/calculation data output as one or more instructions, code or other information on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. The memory storage can also be rotating magnetic hard disk drives, optical disk drives, or flash memory based storage drives or other such solid state, magnetic, or optical storage devices.

Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

Operations as described herein can be carried out on or over a website. The website can be operated on a server computer, or operated locally, e.g., by being downloaded to the client computer, or operated via a server farm. The website can be accessed over a mobile phone or a PDA, or on any other client. The website can use HTML code in any form, e.g., MHTML, or XML, and via any form such as cascading style sheets (“CSS”) or other.

Also, the inventors hereof intend that only those claims which use the words “means for” are to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims. The computers described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation. The programs may be written in C, or Java, Brew or any other programming language. The programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or media such as a memory stick or SD media, or other removable medium. The programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein.

It is also noted that all features, elements, components, functions, acts and steps described with respect to any embodiment provided herein are intended to be freely combinable and substitutable with those from any other embodiment. If a certain feature, element, component, function, or step is described with respect to only one embodiment, then it should be understood that that feature, element, component, function, or step can be used with every other embodiment described herein unless explicitly stated otherwise. This paragraph therefore serves as antecedent basis and written support for the introduction of claims, at any time, that combine features, elements, components, functions, and acts or steps from different embodiments, or that substitute features, elements, components, functions, and acts or steps from one embodiment with those of another, even if the following description does not explicitly state, in a particular instance, that such combinations or substitutions are possible. It is explicitly acknowledged that express recitation of every possible combination and substitution is overly burdensome, especially given that the permissibility of each and every such combination and substitution will be readily recognized by those of ordinary skill in the art.

In some instances entities are described herein as being coupled to other entities. It should be understood that the terms “interfit”, “coupled” or “connected” (or any of these forms) may be used interchangeably herein and are generic to the direct coupling of two entities (without any non-negligible, e.g., parasitic, intervening entities) and the indirect coupling of two entities (with one or more non-negligible intervening entities). Where entities are shown as being directly coupled together, or described as coupled together without description of any intervening entity, it should be understood that those entities can be indirectly coupled together as well unless the context clearly dictates otherwise.

Reference to a singular item includes the possibility that there are a plurality of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “an,” “said,” and “the” include plural referents unless specifically stated otherwise. In other words, use of the articles allow for “at least one” of the subject item in the description above as well as the claims below.

It is further noted that the claims may be drafted to exclude any optional element (e.g., elements designated as such by description herein a “typical,” that “can” or “may” be used, etc.). Accordingly, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or other use of a “negative” claim limitation language. Without the use of such exclusive terminology, the term “comprising” in the claims shall allow for the inclusion of any additional element—irrespective of whether a given number of elements are enumerated in the claim, or the addition of a feature could be regarded as transforming the nature of an element set forth in the claims. Yet, it is contemplated that any such “comprising” term in the claims may be amended to exclusive-type “consisting” language. Also, except as specifically defined herein, all technical and scientific terms used herein are to be given as broad a commonly understood meaning to those skilled in the art as possible while maintaining claim validity.

While the embodiments are susceptible to various modifications and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that these embodiments are not to be limited to the particular form disclosed, but to the contrary, these embodiments are to cover all modifications, equivalents, and alternatives falling within the spirit of the disclosure. Furthermore, any features, functions, acts, steps, or elements of the embodiments may be recited in or added to the claims, as well as negative limitations (as referenced above, or otherwise) that define the inventive scope of the claims by features, functions, steps, or elements that are not within that scope. Thus, the breadth of the inventive variations or invention embodiments are not to be limited to the examples provided, but only by the scope of the following claim language. That being said, we claim: 

1.-120. (canceled)
 121. A method of optical interrogation of a sample material, the method comprising: charging the material at a sample site with radiation to a first charging level; reducing the intensity of the charging radiation; obtaining multiple charge state decay measurements at the sample site by directing an interrogating optical beam at the sample site at different times after reducing the intensity of the charging radiation to provide decay curve data; and determining a characteristic of the decay curve data to determine a parameter of the sample material, wherein the multiple charge state decay measurements are obtained within 1 second after reducing the intensity of the charging radiation.
 122. The method of claim 121, wherein determining a characteristic of the decay curve data includes determining a shape of the decay curve data.
 123. The method of claim 121, wherein the parameter is selected from at least one of: a charge carrier density, a trap charge density, an occupied trap density, a carrier injection threshold energy, a charge carrier lifetime, a charge accumulation time, and trapping cross-section.
 124. The method of claim 121, further comprising obtaining charge state decay measurements at another sample site at different times after reducing the intensity of the charging radiation to provide the decay curve data.
 125. The method of claim 124, further comprising charging the material at another sample site with radiation to a second charging level.
 126. The method of claim 125, wherein the first and the second charging level are same.
 127. The method of claim 125, wherein the first and the second charging level are different.
 128. A system for optical interrogation of a sample material, the system comprising: an optical source capable of charging the material at a sample site with charging radiation to a first charging level; a detection system capable of obtaining multiple charge state decay measurements at the sample site at different times after reducing the intensity of the charging radiation to provide a decay curve data; and a processor capable of determining a characteristic of the decay curve data to determine a parameter of the sample material, wherein the multiple charge state decay measurements are obtained within 1 second after reducing intensity of charging radiation.
 129. The system of claim 128, wherein the detection system is capable of obtaining the multiple charge state decay measurements at the sample site at different times in less than 50 milliseconds after reducing the intensity of the charging radiation to provide a decay curve data.
 130. The system of claim 128, wherein the detection system is capable of obtaining the multiple charge state decay measurements at the sample site at different times in less than 1 millisecond after reducing the intensity of the charging radiation to provide a decay curve data.
 131. The system of claim 128, wherein the detection system is capable of obtaining the multiple charge state decay measurements at the sample site at different times in less than 500 microseconds after reducing the intensity of the charging radiation to provide a decay curve data.
 132. The system of claim 128, wherein the optical source comprises at least one of: a UV flash lamp, a laser and a pulsed laser.
 133. The system of claim 128, wherein the detection system comprises a probe beam; and a detector capable of detecting a second harmonic generation (SHG) signal associated with the probe beam.
 134. The system of claim 133, wherein the probe beam is emitted by a pulsed laser. 